Operating an engine with more than a single fuel allows an engine to operate in a way that may improve engine operation as compared to when only a single fuel type is available. For example, an engine operating with reformed fuel may be able to tolerate a higher level of cylinder dilution than an engine operating solely with gasoline. On the other hand, it may be desirable to operate the engine solely with gasoline when reformate is unavailable or in low supply. Thus, it may be desirable to adjust operation of an engine depending on an amount of available fuel. U.S. Patent Application 2008/0221778 describes a system wherein engine speed and load are set differently when a quantity of fuel stored in a second fuel tank is less than a predetermined value.
While it may make sense to adjust engine speed and load in response to an amount of fuel in a fuel tank, simply limiting engine speed and load may not leverage the remaining fuel in a way that improves engine operation with the remaining fuel. Further, although it may be desirable to limit engine speed and load, such limiting may provide little benefit if the engine is operating at high dilution levels.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for improving engine operation. One embodiment of the present description includes a method for operating an engine, comprising: reforming a first fuel into a gaseous fuel; operating said engine by injecting said gaseous fuel and a second fuel to a cylinder of said engine in response to an available amount of said gaseous fuel, engine speed, and engine load; and adjusting an engine actuator to vary cylinder charge in response to said available amount of said gaseous fuel, said engine actuator further adjusted in response to an amount of octane increase provided to a cylinder air-fuel mixture by said gaseous fuel.
By adjusting cylinder charge amount in response to an available amount of gaseous fuel it may be possible to provide increased engine output for a longer period of time. For example, it may be desirable to reduce the amount of gaseous fuel injected to engine cylinders and to reduce cylinder charge amount from a first amount to a second amount at particular engine operating conditions to conserve gaseous fuel when the amount of gaseous stored in a storage tank is less than a predetermined amount. Lowering the amount of injected gaseous fuel may allow the engine to operate at a load that is higher than when the engine is operated without gaseous fuel. Further, the engine may be operated at a lower load for a longer period of time than if the engine were allowed to continue operating at a higher load.
The present description may provide several advantages. Specifically, the approach may reduce engine emissions and improve fuel economy by better utilizing gaseous fuel at higher engine loads. Further, the method may extend the amount of time that it is possible to operate the engine at higher loads and reduced emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.